My guess is he has developed a habit (snapping) and for all intents and purposes you have too (putting up with it). I'd be inclined to say firmly but not hatefully, at the time he is snapping at you: "I don't appreciate your tone" or "when you are short with me it hurts my feelings" or "No need to snap." I'd repeat it as often as it took for him to change his habit or for me to become desensitized to his habit. Is he able to apologize for other things? Or does he just never apologize for anything? I sincerely believe that the person who can offer a sincere apology where it is needed and who can accept a sincere apology when it is offered are much happier people overall.